Acrylic pressure-sensitive adhesives have excellent tackiness, cohesive force, aging resistance, etc., and have been wisely used. In particular, the demand has recently increased for pressure-sensitive adhesives having high heat resistance as a fixing tape for adhering parts or as masking tape in a soldering step when assembling electronic parts, etc.
An acrylic pressure-sensitive adhesive is generally prepared by copolymerizing an acrylic acid alkyl ester having from 2 to 14 carbon atoms in the alkyl moiety as the main component. If necessary, a small amount of a polar monomer such as acrylic acid, etc., together with monomer component(s) giving a high glass transition temperature, such as styrene, vinyl acetate, etc., for well-balancing the releasing characteristics, tack, etc. Also, for improving the heat resistance and the cohesive force, the adhesive can be crosslinked.
During the production of such an acrylic pressure-sensitive adhesive, usually, after solution polymerizing a monomer in an organic solvent, the polymer formed is compounded with a tackifying resin, a crosslinking agent, etc., to provide an adhesive composition, and after coating the composition on a backing, a large amount of organic solvent is removed by heating to provide a sheet-form or tape-form adhesive sheet.
However, in this generally used method, because of the occurrence of chain transfer into the organic solvent during polymerization, the average molecular weight of the polymer formed is at most about 1,000,000. If the polymer is not crosslinked, the adhesive obtained has an insufficient holding property and even if the polymer is crosslinked, the cohesive force is lowered and holding property is greatly lowered at high temperatures of 100.degree. C. or more. Accordingly, when the adhesive is used in a fixing tape or a masking tape during a soldering step, the problem of slipping and oozing of the adhesive can occur. Also, the low boiling organic solvent used in the production of the adhesive remains, but the remaining solvent is vaporized and expanded at a high temperature to cause the problems of the occurrences of foaming, swelling, slipping, etc., at the adhered surface and also corrosion of electronic parts with the vaporized gas.
On the other hand, recently, from the viewpoint of safety and environmental sanitation caused by the use of organic solvents, photopolymerization type adhesives have been proposed in acrylic pressure-sensitive adhesives. For example, U.S. Pat. No. 4,181,752 discloses a method of obtaining a pressure-sensitive adhesive without using an organic solvent by photo-polymerizing an acrylic acid alkyl ester and a monoethylenically unsaturated monomer on a backing. Also, it is known that a transfer type pressure-sensitive adhesive, which is the same type of adhesive described above, is first formed on a release liner and then transferred onto a backing.
By using the above photopolymerization type adhesives, the above-described various problems caused by the use of organic solvents are wholly overcome and also it is expected that by irradiating the monomer components with light having a relatively weak intensity, the molecular weight of the polymer can be increased and a pressure-sensitive adhesive having a high degree of crosslinking and a large cohesive force is obtained.
However, in the conventional photopolymerization type pressure-sensitive adhesives, since the photopolymerization is carried out in a completely bulk state, the polymerization rate is not 100% by no means and few percent by weight of the unreacted monomers remain. The unreacted monomers thus remaining not only cause a bad smell but also act as a plasticizer of the adhesive that greatly lowers the cohesive force at high temperatures; causes swelling of the adhered surface by the vaporization of the monomers at high temperatures, and causes stains by the gas, all which inhibit the use of the adhesive for heat resistance uses.
For reducing the amount of unreacted monomers, it has been considered to improve the polymerization rate by using a large amount of a photopolymerization initiator, but in such a case, the molecular weight of the polymer formed is lowered with the increase of the initiation radical concentration. As a result, the cohesive force of the adhesive at a high temperature is lowered and a high heat resistance, which is desired in a soldering step, etc., has not yet been attained.
Furthermore, JP-A-2-60981 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes that in a method of producing a pressure-sensitive adhesive tape by a photopolymerization, the photopolymerization is carried out in two separate stages to reduce the amount of residual monomers by improving the photopolymerization rate and improving the productivity.
However, in the above method, to consume the residual monomers in the 2nd stage, it is necessary to irradiate the polymer formed in the 1st stage with light having a higher intensity than light used in the 1st stage, the formation of low molecular weight materials is inevitable in the 2nd stage. Hence although the additive obtained has a heat resistance of about 100.degree. C., in a soldering heat-resistance use at a higher temperature, there are difficulties in forming prints and stains on the adhesive surface of the pressure-sensitive adhesive tape because of the bleeding of the low molecular weight materials peeling, slipping, falling, etc.